1. Field of the Invention
The present invention relates to a method for manufacturing a crystalline semiconductor film that can be used in a semiconductor device, by which an amorphous semiconductor film is irradiated with a laser to be crystallized, and a method for manufacturing a thin film transistor using the crystalline semiconductor film. Note that a semiconductor device in the present specification generally indicates a device capable of functioning by utilizing semiconductor characteristics, and electro-optic devices, semiconductor circuits, and electronic devices are all semiconductor devices.
2. Description of the Related Art
In recent years, it has been possible to form a liquid crystal display device provided with a driver circuit over a cheap glass substrate by crystalline silicon thin film transistors (TFT). As a method for forming a crystalline silicon thin film, a method can be given, in which an amorphous silicon film is irradiated with laser light to be crystallized, thereby obtaining a crystalline silicon thin film.
It is generally known that use of a continuous laser in crystallization of a semiconductor film make a grain size of crystal formed in the semiconductor film to be increased. When a crystal grain size in the semiconductor film becomes large, the number of grain boundaries in a channel region of a TFT that is formed using the semiconductor film is reduced, and mobility is increased, so that the semiconductor film can be utilized for development of high performance devices.
By changing the scanning speed and the spot form of the substrate as appropriate, even when a pulsed wave laser is used, a semiconductor film having large crystal grains can be obtained as similar to the case of using the continuous wave laser.
In a crystallization method of a semiconductor film using a laser, it is known that orientation of crystal is easily aligned, for example, by forming a silicon oxide film with a thickness of about several hundreds nm as a cap film on the semiconductor film that is to be crystallized and performing laser crystallization. Further, it is also known that the cap film makes laser crystallization possible even when the semiconductor film is an ultra-thin film with a thickness of 30 nm or less.
However, in a case where laser crystallization is performed after forming a cap film on the semiconductor film, the cap film is necessary to be removed in a step after the laser crystallization, and the number of steps is increased. In the case of manufacturing TFTs and the like, a step immediately after the laser crystallization step is usually formation of islands by patterning the semiconductor film, and the cap film is necessary to be removed before formation of islands because the cap film becomes hindrance to photo resists and the like. The removal of the cap film is performed in an etching step. A chemical solution such as HF is used in a wet etching method and an etching gas such as CF4 is used in a dry etching, and therefore, the cost is increased caused by increase of steps, and treatment of the chemical solution and gas is needed (for example, Reference 1: Japanese Published Patent Application No. 2000-228360).